These teachings relate generally to frequency sources and oscillators, and more specifically relate to voltage controlled oscillators used in mobile communication devices, in particular voltage controlled oscillators that are constructed as part of an integrated circuit.
A local oscillator (LO) signal is required for receiving and transmitting in a wireless (RF) communication device, such as a cellular telephone. A voltage controlled oscillator (VCO) is typically used in a phase-locked loop to generate the LO signal. An example of the use of a VCO in a mobile station can be found in commonly assigned U.S. Pat. No. 5,471,652, xe2x80x9cFrequency Synthesizer and Multiplier Circuit Arrangement for a Radio Telephonexe2x80x9d, by Jaakko Hulkko. Another example of a VCO used in a mobile station can be found in commonly assigned U.S. Pat. No. 5,926,071, xe2x80x9cMinimization of the Power Consumption in an Oscillatorxe2x80x9d, by Osmo Kukkonen.
In the conventional arrangement the VCO is typically provided as a separate discrete module. However, this results in increased cost and an increase in the required circuit board area to accommodate the VCO module. As such, a desirable goal is to integrate the VCO circuitry into an existing integrated circuit package with other radio frequency (RF) circuitry, thereby reducing the cost and the required circuit board area.
However, a problem that arises when attempting to integrate the VCO circuitry is a resultant increase in the circuit noise level. An examination of this problem reveals that one significant noise source is within the VCO biasing circuitry. Filtering this noise has proven to be difficult.
FIG. 1 is a circuit diagram of a conventional RF oscillator circuit 1 that forms a part of a VCO. The oscillator circuit 1 includes an oscillator portion 2 (formed of cross-coupled transistors MP3 and MP4 and a resonator network 2A), and a bias portion 3 (formed of diode-connected transistor MP1 and current source transistor MP2, and a bias generator 3A).
The above-noted noise problem results from the fact that transistors MP3 and MP4 operate in a non-linear manner, as they are essentially switching transistors. MP3 and MP4 therefore are capable of mixing low frequency noise into the oscillator output frequency. If the biasing circuitry 3 also generates noise that appears at the output of the current source (at the drain of MP2), then the overall noise floor of the oscillator 1 is raised. Because of its low frequency, the noise generated by the biasing circuitry 3 is difficult to filter out. In practice, large inductance and capacitance values would be needed in order to adequately filter the low frequency noise. The required physical components would, however, be difficult or impossible to incorporate within an integrated circuit, and thus additional pins would be required for connecting to the external oscillator bias filter components. As can be appreciated, the requirement to provide the external VCO low frequency filter components would defeat the purpose of integrating the VCO circuitry.
FIGS. 2A and 2B illustrate two potential solutions to this problem, however neither is adequate when designing an oscillator to exhibit exacting tolerances and operational specifications, as are commonly required in modem cellular communications systems. More particularly, FIG. 2A shows the use of a simple (low inherent noise) resistor for the bias generator 3A, while FIG. 2B shows a constant current source embodiment. The approach in FIG. 2A is undesirable as the oscillator frequency can easily drift due to changes in the operating voltage or temperature, while the approach of FIG. 2B, using two parallel current sources, generates additional noise within the bias circuitry 3.
It should be noted that in conventional practice there can be two inputs to a VCO, one for biasing a capacitance diode (CD) to set the operating point of the capacitance diode into a linear region, and a second for tuning the VCO resonant frequency. Reference can be had to FIG. 8A, which is based on FIG. 1A of commonly assigned U.S. Pat. No. 5,764,109, xe2x80x9cVoltage-Controlled Oscillator (VCO) Having a Voltage Derived from its Output to Tune its Center Frequencyxe2x80x9d, by Osmo Kukkonen. In FIG. 8A the input Vcontrol (Vcrl) is typically obtained from a phased locked loop (PLL) and is used to tune the VCO resonant frequency according to a selected receiving/transmitting channel. Vcrl is typically adjusted or preset so that it is in the middle of the tuning range when the PLL is tuned to the center frequency channel. The second input (Vcf) may be provided for improving the linearity of the capacitance diode (CD, typically a varicap) such that Vcontrol (Vcrl) acts on the linear portion of the CD adjustment range. Vcf may thus be considered to function in a manner analogous to a bias voltage, at least with respect to CD. As Vcf changes the voltage between the terminals of CD it has some effect on the VCO resonant frequency, but this is typically a preset, and not a continuously variable frequency effect. The use of Vcf is optional, and depends at least in part on the characteristics of the capacitance diode and on the Vcrl tuning value and range.
Also shown in FIG. 8A is the bias generator 3A and its connection to the active part of the oscillator portion 2, specifically the cross-coupled oscillator transistors MP3 and MP4. The bias generator 3A is provided to set the operating point of the oscillator transistors MP3 and MP4, and generally will have little or no effect on the output frequency of the VCO. However, and as was discussed above, the low frequency noise output by the bias generator 3A can be mixed into the output of the VCO, and can thus appear in the RF output (RFout) of the VCO shown in FIG. 8A.
Reference with regard to VCO circuitry can also be made to commonly assigned U.S. Pat. No. 5,859,573, xe2x80x9cCircuitry for Separating the Output of an Oscillator from the Other Parts of a Mobile Communication Systemxe2x80x9d, also by Osmo Kukkonen.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
Disclosed is an oscillator circuit for use in a local oscillator of an RF communications device that communicates over an RF channel. The oscillator circuit includes an oscillator circuit portion and a bias circuit portion coupled to the oscillator circuit portion for setting the operating point of the oscillator transistors. The oscillator circuit further includes a switch for selectively one of connecting or disconnecting the bias circuit portion from the oscillator circuit portion, and a capacitance for storing an output of the bias circuit portion during a time that the switch disconnects the bias circuit portion from the oscillator circuit portion, thereby maintaining control of the operating point of the oscillator transistors of the oscillator circuit portion. When the switch is open noise generated by the biasing circuit portion is prevented from reaching the oscillator circuit portion, thereby reducing the overall noise floor of the oscillator circuit.
In a presently preferred embodiment the bias circuit portion is constructed so as to include a current mirror transistor that is connected to a gate terminal of a current source transistor, where a drain terminal of the current mirror transistor is coupled to the oscillator circuit portion, and where the switch is interposed between the current mirror transistor and the gate terminal of the current source transistor. In this embodiment the capacitance is coupled between the gate terminal of the current source transistor and a source terminal of the current source transistor.
In accordance with an aspect of this invention, the capacitance may have a value that is measured in tens of picofarads, and the oscillator circuit, including the capacitance, is formed within an integrated circuit.
The switch may remain open for an extended period of time when operating in the same frequency band at about the same temperature (e.g., where there no operational changes and little or no environmental changes), as the capacitance is capable of storing the desired charge for long periods of time.
In one non-limiting embodiment the switch is opened during an RF burst that is received by the RF communications device from the RF channel, or the switch is opened during an RF burst that is transmitted by the RF communications device to the RF channel, depending on whether the oscillator circuit is used in the receiver or the transmitter, respectively, of the communications device.
More specifically, the switch is opened during periods when the mobile station is demodulating a received signal or modulating a signal to be transmitted, as lowest noise operation is desired at these times. At other times the switch can be closed to refresh or update the charge on the capacitance.